'Politicians say they don't look at the ­opinion polls – bullshit! Of course they do! We all do!" David Cameron exclaimed at one of his meet-the-people ­sessions this month. And he is right. Polls are not just a symptom of politics, a fever or a chill, but the difference between health and disease. A shudder in the Labour ­rating? The cabinet starts murmuring. The Tories slide below 40%? Cameron had better find kind words to say about Nick Clegg. Greens up; Ukip down; BNP anywhere – the predictive magic of numbers ­quantifying the thoughts of 45 million voters into 100 tidy units.

Polls are why we think Labour is going to lose this spring. Polls are why there was no election in 2007. Polls are (in part) why Cameron is and stays leader. But before we trust polls, we should ask how they are created and whether they might be misleading. What if the British political class is ­chasing digits without substance?

The Guardian commissions a poll at least every month, and at its core are voting intention figures that matter more and are calculated differently from the answers to any other questions. Our partnership with ICM runs back to 1980 and the record – the Great Polling Disaster of 1992 apart – has been excellent.

In 2005 most firms got the vote shares almost spot on. In America, they ­predicted President Obama. This winter, the broad outline of national opinion described by polls is surely right – an unpopular Labour government, a more popular (but not unshakable) ­opposition and an electorate that doesn't like ­politicians of any kind.

It is likely that the polls are accurate. But the possibility of some serious and sustained fault is not negligible. Part ­science, part art, polling is as accurate as its last mistake.

A typical British poll includes the views of something over 1,000 voters – 1,000 being the statistical point at which the margin of error reduces to close to 3%. The tricky thing about polling voting intention, however you do it, is that it is no use asking people what they plan to do and just totting up the totals. When it comes to the ballot box, some of the people contacted won't vote. Others might not have told the truth. And few of us know for sure what we will do. Further, the kind of people who find the time to answer questions from a stranger calling their home, or who join an internet panel, will not be representative of the public as a whole. Pollsters know this, of course; and know they must weight figures to reflect the likely characteristics of the electorate. And this is where the difficulties begin.

For the past 25 years telephone pollsters have been wrestling with a ­persistent tendency of the polls to overstate Labour's share of the vote. Since 1983 the only final poll before election day to have proved to be too kind to the Conservatives was ICM's in 1997. Pollsters have got used to voters treating the Conservatives as the underdogs. They use adjustments to allow for the fact that telephone samples contain more Labour voters than the electorate as a whole, and that Labour voters overstate their likelihood to vote.

They have also had to deal with the so-called spiral of silence – the fact that some voters whose party choice is unpopular will hide their party allegiance from pollsters. Plot election polls for the last 25 years on a chart, and most overstate the likely lead of the winning party. All this has had the effect of increasing figures for Tory support – and making the polls more accurate.

But what if, in the context of 2010, these assumptions are turned on their head? We already know that in the last 18 months or so the spiral of silence has helped Labour: pollsters now find themselves having to increase Labour's share, to take account of people who say they have switched away from the party but may still turn out for it on polling day. Perhaps some shy Labour voters are even now evading the pollsters' radar.

By correcting the error of 1992 – when Labour support was put too high – polling could have set itself up for a new fall in 2010, by putting it too low. The opposite might be true, too – that the spiral of silence is not working in Labour's favour at all. If this is the case the polls may, yet again, be too generous to the party.

Evidence of error either way is thin. It would be irresponsible for pollsters to ditch methods that proved so ­accurate in 2005, on a hunch that next time things might be different. But the interposition of weighting, between the raw data and the published poll, includes what is essentially a kind of expert hunch. Last year brought a poll every five days or so, from half a dozen companies using different weightings and research techniques. Their results were strikingly close. There is pressure in numbers, and no polling firm wants to break ranks without reason to do so.

There is an obvious commercial desire to be the most accurate, but heavy commercial risk arises from being the most wrong. Pollsters have to wonder whether it is better to be in with the herd. That is fine if they herd around an accurate consensus view, as in 2005 – but look what happened in 1992. That the polls all say the same thing does not in itself mean they are right. When weary Labour ministers insist that the election is not yet lost, little as they may believe it, they may have a point.